The position and control of rotational machinery typically requires monitoring both the rotational position as well as the rotational velocity. These measurements are usually provided by separate tachometers and synchro sensing elements. The connection of multiple-shaft monitoring devices requires complex and failure-prone mechanical assemblies, which of course increase cost and difficulty of installation while reducing reliability.
Futhermore, tachometers and synchros typically comprise a rotatable armature having electrical signals produced by or required for energization of the armature. The electrical signals on the armature must be transferred by an electrical or a mechanical means, such as slip rings or a transformer assembly. This additional structure further decreases reliability and increases costs.